The performance of sensors, such as displacement sensors, is frequently degraded by hysteresis. Displacement sensors may be absolute sensors such as certain inclinometers and accelerometers that measure the inclination or acceleration of the housing or enclosure of the sensor with respect to the earth or other inertial reference frame. Displacement sensors may also be used to measure the relative displacement between two or more points. Such sensors may be physically connected to such points or may rely on, for example, magnetic or electric fields or electromagnetic or acoustic waves to link to such points.
Displacement sensors typically comprise a base and certain sensing elements, within a sensing unit, that are immovably connected to the base. Displacement sensors, such as for example inclinometers, also contain certain sensing elements, within the sensing unit, that may move relative to the base as a result of motion that is imparted to the base. The relative motion between these two types of elements within the sensing unit is typically measured and used to determine the displacement of the base.
If an ideal error-free displacement sensor, such as a single axis inclinometer with sufficient range and without hysteresis, underwent exactly a 25 degree clockwise change in inclination about its sensitive axis followed by a counterclockwise change in inclination of exactly 25 degrees about the same axis, the sensor would indicate a net change in inclination of precisely zero degrees. However, due to hysteresis, conventional displacement sensors typically cannot perform in this manner.
The present applicant was a co-inventor of an invention described in U.S. Pat. No. 4,624,140 the contents of which are included herein by reference in their entirety. An inclinometer disclosed in that patent comprises a sensing unit comprising a spherical vessel, partially filled with a conductive liquid, with conductive wall segments at least one of which is coated with a thin dielectric coating. In use, when the inclination of such an inclinometer is varied, the conductive liquid covers a variable portion of at least one dielectric coated wall segment. The capacitance between the conductive liquid and the coated wall segment varies as function of the inclination of the base of the device. An alternate capacitive sensor, which uses a low conductivity liquid as the dielectric of a capacitor, is disclosed in U.S. Pat. No. 3,906,471, the contents of which are included herein by reference in their entirety. U.S. U.S. Pat. Nos. 4,912,662 and 5,083,383, the contents of which are included herein by reference in their entirety, also describe other configurations of inclinometers. Generally, the accuracy of inclinometers and other displacement sensor technologies, with and without liquid sensing elements, are limited by hysteresis.
The sensing units in displacement sensors typically have components that are immovably fixed to the housing or base of the sensor and others that are free to move or have the propensity to move relative to the housing or base when the sensor is displaced. The relatively fixed elements in the sensing unit of the inclinometer disclosed in U.S. Pat. No. 4,624,140 comprise the vessel and the conductive wall segments. The conductive liquid, on the other hand, is a movable element within the sensing unit that moves relative to the housing of the sensor or the sensor base when the housing and base are displaced.
Displacement sensors are typically configured to be sensitive to a single input. For example, an inclinometer is typically configured to measure only changes in inclination of its base. Although, a two dimensional sensor may be used to measure an inclination change in two dimensions, the only input that can typically be measured with such a device is change in inclination of the base or housing.
U.S. Pat. No. 1,637,445, the contents of which are included herein by reference in their entirety, describes the use of a liquid filled, shaft mounted, variable capacitor attached to a turning knob of a radio. Such a device cannot be used as an inclinometer because the output of the variable capacitor is sensitive to two different inputs, namely the inclination of the base of the radio and the rotation of knob 25 in FIG. 1 of the patent. In such a device, the output of the capacitance is the result of an indeterminate combination of the inclination of the base and the rotation of the knob.